[lwip-users] TCP_WRITE_FLAG_MORE, tcp_write(), tcp_output() and real output TCP segment

[Giuseppe Modugno]

I'm trying to understand the mechanisms under TCP_WRITE_FLAG_MORE, tcp_write() and tcp_output(). I'm using raw API.

First of all, I couldn't understand if tcp_output() is useful or not. I understood tcp_output() *really* outputs unsent data, pushing all data to netif driver. Here it's not clear:

• if unacked data (data already sent but not yet acknlowedged) is sent too;
• what happens if data in the output buffer is bigger than what netif driver could accept in its sending function (maybe a well written application checks tcp_sndbuf() before writing output data, so this event never happens).

For example:

tcp_write(...)
tcp_write(...)
tcp_write(...)
tcp_output()

Is tcp_output() really needed? It seems data are send even without calling tcp_output().

Another question is the correspondence between tcp_write() calls and output TCP segments, IP packets and Ethernet frames. Are three TCP segments, three IP packets and three Ethernet frames generated? Does lwip try to compact output data in a single TCP segment, IP packet and Ethernet frame?

Another question is about TCP_WRITE_FLAG_MORE flag. I understood this flag should be set in all tcp_write() except the last. However it seems it works with and without that flag in sending data.